Mechanical drug and agent delivery devices are utilized in a wide range of applications including a number of biological applications, such as catheter interventions and other implantable devices used to create a therapeutic or other biological effect within the body. Often, such delivery devices take the form of radially expandable devices used to mechanically open an occluded or narrowed blood vessel or body fluid channel or cavity. For example, inflatable non-elastomeric balloons have been utilized for treatment of body passages occluded by disease and for maintenance of the proper position of catheter-delivered medical devices, such as stents, within such body passages. With the use of both permanent and bioerodible drug carrying polymers applied directly onto the radially expandable stents to form drug eluting medical device, such devices are placed within body lumens with drugs or agents embedded therein for release of the drug or agent within the body over some pre-determined or extended period of time.
Some intervention balloon catheters are made to deliver a systemic bolus of liquid or gas that includes a drug, to a targeted tissue location within the body using an open catheter lumen or channel located at some length along the catheter shaft. Unfortunately, when such systemic delivery means are used to deliver a controlled volume of medication to a desired tissue location, a majority of the medication is lost to systemic circulation when the balloons are deflated following drug delivery and reperfusion by the body fluid or blood washes the infused deliverable away. The overall inefficiency of injectable fluids required for most small diameter catheter lumen devices lies in the dilution of the localized therapeutic agents just to pass through the length of small diameter catheter shaft. Such liquid dilution carrying mediums often prevent and limit the amount and rate at which the localized uptake of the intended therapeutic drug can penetrate local tissue after reperfusion is established upon catheter removal. Generally, most liquid formulations containing a drug or agent that is delivered to the targeted tissue location by liquid bolus does not penetrate the tissue sufficiently at the targeted tissue location to result in a significant therapeutic effect, and is consequently washed away by body fluids. This post drug delivery systemic dilution substantially diminishes the effectiveness of the drugs or agents provided through such delivery devices, and increases the likelihood of a greater systemic effect caused by the large quantity of drug or agent washed into the bloodstream. To compensate for such delivery inefficiency, the dose of drugs or agents must be volumetrically increased in anticipation that they will be principally washed away before therapeutically effecting the localized or targeted tissue area. However, because of the risk of increased systemic effects and possibly toxic overload, the volume of the drugs or agents must not exceed that which can still be considered safe for exposure by systematic dilution and subsequent systematic distribution throughout the patient's body. The drug or agent used in such an intervention delivery method must be safe enough in its diluted state to be washed away to other parts of the patient's body and not have unwanted therapeutic or otherwise detrimental effects. There is a delicate balance between making the drugs or agents sufficiently concentrated to have therapeutic characteristics at the targeted tissue location, while also being sufficiently diluted to avoid harmful effects after being washed away into the body's systemic circulation.
A further drug and agent delivery vehicle conventionally includes drug eluting stents. It is has been demonstrated that the localized concentration of drug permeation into tissue varies with the existing stent delivery vehicles, depending upon the drug load, drug dose, and release profile of such polymeric stent coatings used to carry and release the therapeutic agents after permanent stent device deployment. The drug concentrations at the struts of the stents are relatively higher than drug concentrations at areas between the struts. This can adversely affect the uniformity and localized therapeutic effect of the drug. More specifically, the drug concentration can be too high in some areas of the tissue, while the drug concentration in other areas can be inadequately efficacious. Furthermore, the distribution of the drug by a coated stent to the tissue occurs only along the struts of the stent. If the generally cylindrical shape of a stent represents a total surface area of 100%, the actual location of the struts that form the stent after expansion typically represents less than 20% of the surface area of the total cylindrical shape. Even if the surface area of the struts represented greater than 20% after radial expansion, the remaining portions of the cylindrical shape still would remain porous with a majority of large openings in the cylindrical stent geometry. The drug can only be transferred in those locations where the struts exist. Thus, with a conventional drug containing coated stent there are large sections where the drug cannot exist and cannot make direct contact with the tissue. After conventional drug eluting stent deployment, wherein a first small diameter slotted tube is inserted into the targeted organ space and expanded to a larger second diameter, the slotted tube becomes mostly open during the strut plastic deformation. Therefore, the large open sections of a deployed stent do not provide any means for delivering medication between the struts, or any means for the drug to be transferred into the tissue.